Chalkboard compass



Nov. 26, 1963 G. A. MCALlsTER, JR 3,111,761

CHALKBOARD COMPASS Filed March 27. 1961 /l/l/l/lI/llllllllll/Ln l INVENTOR y George A. McAlis1er,Jr.

BY M2M ATTORNEY United States Patent O 3,111,761 CHALKBARD CGMPASS George A. lVicAlister, lr., Big Spring, Tex., assigner to Math-Master Labs, incorporated, Eig tfipring, rEex. Filed Mar. 27, 1961, Ser. No. AtZ 1 Claim. (Cl. 33-27) This invention is concerned with a drawing instrument, and is particularly concerned with a compass and chalkholder combination arranged to be attached to a blackboard or like surface for drawing a circle or arc thereon.

In teaching mathematics or for demonstration purposes, is is often necessary to draw a circle or arc on a chalkboard, wall surface, or chart, and no satisfactory instrument has been heretofore devised for accurately carrying out such operation in drawing large diameter circles.

A common method of describing a large diameter arc or circle on a blackboard or other surface with chalk is to tie a piece of chalk to the end of a string, place the iinger on the string, and press it to the chalkboard to provide a center point for the circle or arc, and draw the circle or arc about the center point so provided. lt is most difricult to draw an accurate circle or arc by such means, as the iinger often slips or the string is engaged with the `finger as the chalk -is moved about, to shorten the radius of the circle drawn. Furthermore, the radius of the desired circle to be drawn cannot be accurately me-asured and adjusted. Furthermore, the radius of the desired circle cannot be accurately predetermined by such method.

The invention disclosed and claimed herein includes a chalk holding blackboard compass, including a base member which may be secured to the chalkboard or other surface by suction cups, and an arm rotatably disposed about the base member, carrying a chalkholder, which is extensibily adjustable on the arm, so as to provide a rm anchor for the chalk compass, and at the same time provides means for adjusting the radius of the circle desired to be drawn.

Perpendicular centering lines are provided on a transparent base, for aligning the center with coordinate lines on a graph form. The arm is hinged to the base so that the chalk may ride over irregularities, or spaces may be skipped in the arc by simply lifting up on the chalkholder; and graduations are provided on the chalkholder arm to provide visual adjustment of the radius of the circle or arc.

The invention also includes a novel chalkholder slidably disposed on the compass arm, which has a bore therethrough, including a counter bore, with chuck jaws arranged thereabout to grip and hold the chalk therein, which permits la pencil or other elongated object to be inserted through the bore to dislodge pieces of broken chalk therefrom.

It is, therefore, a primary object of the invention to provide a drawing instrument for drawing circles or arcs on a chalkboard, chart, wall, or other surface, with which an accurate circle or arc can be drawn of :a selected radius.

Another object of the invention is to provide a drawing instrument for describing an arc or circle which may be detachably and securely attached to the surface on which the arc or circle is to be drawn, and accurate-ly positioned thereon, with a center at a predetermined selected point, and with a radius of a selected length.

Still another object of the invention is to provide a combination compass and chalkholder attachable to a surface on which an arc or a circle is to be drawn with a novel chalkholder slidably and detachably disposed on the arm of the compass in adjusted relationship thereon, to accurately draw a circle or arc with an exact center point lwith a predetermined and selected radius.

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Still another object of the invention is to provide a compass for drawing a circle or arc on a chalkboard or like surface, which includes a base having suction cups thereon to detachably secure the base to the surface on which the arc or circle is to be drawn, and an arm rotatable about the center point of the base, and being hingedly attached for vertical movement with relation to the base, and a chalkholder frictionally slidable on the arm for adjustment of the radius of the circle or arc, and means provided in connection with the chalkholder to disengageably hold the chalk therein in adjusted relationship thereto, the chalkholder having a central bore therethrough, through which instruments such as a pencil may be extended for ejecting broken pieces of chalk therefrom.

A still further object of the invention is to provide such a compass device, fas hereinbefore indicated, having a transparent base thereon, with perpendicular lines intersecting at the center thereof, and disposed under the center of the pivot points of the arm, so that the center of the pivot point for the arm can be exactly positioned over the intersection of any two coordinate lines on a graph form or the like.

Other and further objects of the invention will become apparent upon reading the detailed specifications herein- `after following, and by referring to the drawings annexed hereto.

A suitable embodiment of the Iinvention is shown in the attached drawing, lwherein:

FIGURE l is a top plan view of the circle or arc drawing instrument;

FGURE II is a side elevational view showing the base and pivot connection for the arm in cross-sectional elevation, and showing the chalkholder quarter-sectioned in elevation;

FIGURE -llI is a fragmentary, elevational view of the side of the chalkholder showing the arm extending therethrough, as taken on the line Ill-HI of FIG. II; and

IFIGURE IV is a fragmentary side view of the arm showing the knurled surface thereon to provide frictional engagement of the arm with the transverse passage through the chalkholder body.

Numeral references are employed to indicate the various parts shown in the drawings, and like numerals indicate like parts throughout the various figures of the drawing.

The numeral 1 indicates a base plate which is preferably made of transparent plastic material to which is attached a plurality of radially spaced suction cups 2., by means of bolts 3, the heads of which are preferably molded and embedded into the suction cup body. A threaded bolt 4 extends upwardly through the center of base plate 1, with the head thereof recessed in the lower side of the base.

A nut 5 is turned downwardly against a washer 6 to securely attach the bolt 4 to the base plate 1.

A tubular spacer sleeve 7, which may be made of plastic or metallic material, surrounds the bolt 4 and extends over the nut 5 and washer 6.

A pivot lug yoke 'S has a hole 9* therethrough which is arranged to receive the extension 13 of a cap nut 11. The cap nut 11 is provided with a threaded socket 12. centrally thereof, into which the upper end of the threaded bolt 4 is threadedly engaged when the extension 13 is rotated into the hole 9 in the lug 8. Adequate clearance is provided between extension 13 and hole 9 to provide freedom of rotation therebetween. The extension 13 has a reduced extension 14 thereon which extends downwardly into the sleeve 7, which provides a shoulder 14a to engage the upper end of the sleeve 7 to provide a bearing surface. The lower edge of the hole 9 also engages the upper end of the sleeve 7, and an annular shoulder lla is provided about the head ill, which is arranged to engage the upper end of the hole 9 to provide a bearing surface.

Thus the lug S is rotatable about the extension t3 on the head 11, providing a pivot connection therefor'.

A pair of spaced ears l are provided on the lug 3 between which are inserted the inner end of the arm i6. Aligned holes are provided through the ears i5' and the end of the arm le through which may be extended a bolt 17 for pivotally securing the arm lo between the ears i5.

A chalkholder, generally indicated at lb, is slidably disposed on the arm i6. The chalkholder body liti may be made of plastic material, or other suitable material, and has a longitudinal bore therethrough, including the upper bores 25 and the counter bore i9 in the lower end thereof. The lower end of the body about the counter bore li9 is slitted, as indicated at 2i, to provide springlike segments Ztl.

A piece of chalk 24 may be slipped into the counter bore i9 and abutted against the shouider 19a provided at the upper end of the counter bore, and a slidable metallic ring 212 is slipped downwardly over the tapered surface 23 on the lower end of the body 11S, to contract the segments 2d inwardly to thereby grip the chalk in the counter bore i9. The body lid has a generally rectangular lateral passage 25 therethrough, through which the arm i6 extends. The arm le preferably has a frictionai sliding engagement through the passage 25, and the lower side of the arm le is preferably provided with a knurled surface 27 to provide frictional engagement. Such slidable, frictional engagement is provided so that when the chalkholder body iS is slid to a desired position on the arm i6, it will normally remain in such position until force is exerted thereagainst to move same along the arm lo.

The arm i6 has spaced graduations ZS thereon, so that the chalkholder ib may be slid along the arm at the correct graduated position from the center point of the base 1, in order to draw a circle or arc of the desired radius. The spaced graduations 2% may be viewed through the bore 26 so that the axis of the bore may be aligned with a selected graduation on the arm to thereby align the axis of the chalk 24 with the selected graduation for drawing a circ-le or are of a desired radius.

The transparent base It has perpendicular lines 29 arranged thereon, so that the extension would intersect at the exact center of the base l whereby the lines 29 may be superposed on two intersecting coordinate lines on a graph form provided on a blackboard or chart, in order to exactly position the center of the base at the intersection point of the coordinate lines, so that an accurate arc may be described by the chalk 24 at the correct radius from such intersecting point.

The operation and function of the device hereinbefore described is as follows:

The base i is attached to a blackboard, chart or other surface on which it is desired to draw a circle or an arc with the center point at the desired location, the suction cups 2 are pressed against the surface of the chalkboard or other surface so that they tightly adhere thereto, providing a firm anchor for the compass device. The chalkholder 18 may be slid along the arm 16 to a point to give a desired radius for the circle or arc to be drawn, and it will remain at such point by reason of the frictional engagement of the arm i6 in the hole 25. The circle is then drawn by simply rotating the arm 16 about the reduced portion i3 of the cap `il, with the chalk 2f?.- in engagement with the surface. The chalk will describe an accurate circle or arc.

The chalkholder 18, although being a part of the combination including the base and the arm i6, as used,

can be slipped off the end of the arm 16 and thus can be used separately as a chalkholder. lit provides a very convenient chalkholder for the instructor or student. lt not only provides an elongated gripping surface, similar to a large pencil, but it protects the hand of the user against chafing and cha kdust, and it saves chalk in that the major portion of the chalk is recessed in the counter bore i9, providing a lrm and braced support therefor, so that the chalk will not easily break.

When the chalk 2d is worn away and used up to outer end of the counter bore i9, or if it is broken off even with the lower end of the body 13, it may be removed, by simply sliding the ring 22v upwardly to allow the jaws 20 to spring outwardly. ln the event the chalk becomes stuck and lodged in the counter bore 19, it may be easily removed therefrom by simply passing a pencil or like object through the bore 26 to force the chalk therefrom.

It will be understood that the holder 18 could be used to hold other types of marking instruments, such as pencils, brushes, felt pad markers, etc.

it will be seen that l haves provided a novel and useful drawing instrument including a compass to be used on a chalk board or other surface, which provides a firm attachment to the surface, and which may be positioned at an exact center point for the arc or circle, and the radius may be accurately adjusted so that a perfect circle may be drawn with ease and simplicity; and l have provided a chalkholder in conjunction therewith which has great utiiity and may be used separately to protect the hand of the user against chafing or chalkdust and from which broken pieces of chalk, which have lodged therein, may be easily removed for replacement.

lt wiil be understood that other and further forms of my invention may be devised without departing from the spirit and scope of the appended claim.

l claim:

ln a device of the class described, a transparent base; a plurality of lines provided on the base, the projections of which intersect at right angular relationship at the center of the base; atleast one suction cup attached to the base for securing same to a surface; an arm rotatably attached to the base; a marking instrument holder having a longitudinal bore extending therethrough; a lateral passage through the holder intersecting the bore, and slidably and frictionally extending about the arm; spaced measurement graduations on the arm viewable through one end of the bore to permit visual alignment of a selected graduation with the axis of the bore; a spaced longitudinal siit provided in the wall of the holder, providing springlike segments thereabout; a tapered surface on the holder on the outer side of the segments; a slidable ring about the holder arranged to contract the segments about a marking instrument in the bore when slipped downwardly on the tapered surface; and a knurled surface on the arm frictionally engaging the inner surface of the lateral passage to allow movement on the arm but maintaining the marking instrument holder in a selected position there- References Cited in the lile of this patent UNITED STATES PATENTS 1,583,223 Thierry May 4, 1926 2,355,585 Halstead Aug. 8, 1944 2,570,328 Dritz Oct. 9, 1951 2,656,605 Schl-ieder Oct. 27, 1953 2,767,893 Latson Oct. 23, 1956 FGREEGN PATENTS 198,743 Switzerland Sept. i6, 1938 

